Semiconductor wafers such as silicon and gallium arsenide wafers and various types of packages (which may be collectively referred to as “cut object(s)” herein) are manufactured with a large diameter, and these are cut and separated (diced) into small element pieces (referred to as “chips” hereinafter) before being transferred to a mounting process as the subsequent process. At that time, the cut objects such as semiconductor wafers are preliminarily applied to a pressure-sensitive adhesive sheet to be supplied to various processes including dicing, washing, drying, expanding, picking-up, and mounting.
Heretofore, during processes from the dicing process for cut objects to the picking-up process, a dicing sheet has been used which is comprised of a base material film and a pressure-sensitive adhesive layer formed thereon. In such a dicing sheet, a polyolefin based film, a polyvinyl chloride based film or the like is typically used as the base material film.
At the time of dicing, however, a part of the pressure-sensitive adhesive layer or the base material film may also be cut with each cut object, and dicing dust may arise from the dicing sheet thereby to contaminate chips to be obtained.
One of methods for performing dicing is a full cut dicing, in which a rotating round blade fully cuts the cut object such as a semiconductor wafer while possibly cutting a part of the pressure-sensitive adhesive layer or the base material film of the dicing sheet. At that time, a part of the base material film may be molten due to heat caused by friction with the rotating round blade and elongated to generate string-like dust on a dicing line. Further, such string-like dust still remains with pressure-sensitive adhesive.
If each chip is sealed in a package while being attached thereto with a large amount of the above-described string-like dust, then the string-like dust will be decomposed by heat used for the enclosing thereby to destroy the package or cause operational failure in a device to be obtained. String-like dust with pressure sensitive adhesive is difficult to be removed by washing, and the yield will thus be significantly reduced. Therefore, when performing dicing by using a dicing sheet, it is required to prevent such string-like dicing dust from being generated.
Further, when dicing a package sealed with cured resin as a cut object, dicing is performed using a thicker dicing blade with a deeper cutting depth than the case of dicing a semiconductor wafer. Thus, in such dicing for a package as a cut object, considerable amount of string-like dicing dust may be generated because the amount of the base material film to be cut is large.
To this end, Patent Document 1 discloses an invention which uses as a base material film of a dicing sheet a polyolefin based film having been irradiated to electron beam or γ (gamma) ray with 1 to 80 Mrad. In that invention, resin as a constituent of the base material film appears to be cross-linked through the irradiation to electron beam or γ ray thereby to suppress the dicing dust from being generated.
Patent Document 1 illustrates, as materials for the polyolefin based film to be irradiated to electron beam or γ ray, resins such as polyethylene, polypropylene, polymethylpentene, ethylene-vinyl acetate copolymer, ethylene-(meth)acrylic acid copolymer, ethylene-methyl(meth)acrylate copolymer, ethylene-ethyl(meth)acrylic acid copolymer, ethylene-ionomer copolymer, ethylene-vinyl alcohol copolymer, and polybutene.